Class 12 Physics Electromagnetic Induction Notes for IIT JEE & NEET

Electromagnetic induction is the process by which a changing magnetic flux produces an EMF in a circuit. These class 12 physics electromagnetic induction notes cover the full NCERT chapter and the numerical-based topics asked in JEE and NEET. Read the concepts and formulas below, then use the handwritten pages to practise.

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What Is Electromagnetic Induction?

Electromagnetic induction is the phenomenon of generating an EMF (and a current in a closed circuit) when the magnetic flux linked with the circuit changes. Michael Faraday demonstrated it experimentally in 1831. It is the principle behind generators, transformers and induction motors.

Why Does EMI Happen?

A current is induced whenever the magnetic flux linked with a closed circuit changes with time. The change can arise because:

• The magnitude of the magnetic field B changes.
• The area of the loop changes (e.g., a sliding rod on rails).
• The angle between B and the area vector changes (e.g., a rotating coil).

Key Terms to Know Before You Begin

Magnetic Flux: Definition and Formula

Magnetic flux through a surface measures the magnetic field lines passing through it.

$$\Phi = \vec{B}\cdot\vec{A} = BA\cos\theta$$

Here $B$ is the magnetic field (T), $A$ is the area (m²) and $\theta$ is the angle between $\vec{B}$ and the normal to the surface. The flux is maximum, $BA$, at $\theta = 0^\circ$, zero at $\theta = 90^\circ$ and $-BA$ at $\theta = 180^\circ$.

The SI unit is the weber, where 1 Wb = 1 T m² = 1 V s. In CGS, 1 Wb = $10^{8}$ maxwell. The dimensional formula is $[ML^2T^{-2}A^{-1}]$.

Draw the area vector first and then find its angle with the field. This avoids most sign errors in induced EMF problems.

Faraday's Laws of Electromagnetic Induction

What does Faraday's First Law state?

Faraday's First Law states that whenever the magnetic flux linked with a circuit changes, an EMF is induced in the circuit. The induced EMF exists only as long as the flux is changing — it disappears the moment flux becomes constant.

What does Faraday's Second Law state?

The magnitude of the induced EMF equals the rate of change of flux. For a coil of $N$ turns:

$$\varepsilon = -N\dfrac{d\Phi}{dt}$$

The negative sign comes from Lenz’s law. To find the EMF, write the flux at time $t$, differentiate it with respect to $t$, multiply by $-N$, and use Lenz’s law for the direction.

Faraday’s law is the core of class 12 physics electromagnetic induction notes, so practise the example below until the steps are routine.

Example: a coil of 100 turns and area $0.02\ \text{m}^2$ is perpendicular to a field that falls from 0.5 T to zero in 0.1 s. Then $|\varepsilon| = N\dfrac{A\,\Delta B}{\Delta t} = 100 \times \dfrac{0.02 \times 0.5}{0.1} = 10\ \text{V}$.

Lenz's Law and Conservation of Energy

In class 12 physics electromagnetic induction notes, Lenz’s law states that the induced current flows in a direction that opposes the change in flux that produces it. This is the reason for the negative sign in $\varepsilon = -N\dfrac{d\Phi}{dt}$.

How Is Lenz's Law a Consequence of Energy Conservation?

If the induced current aided the change in flux instead of opposing it, the current would increase the flux, which would further increase the current, creating energy from nothing. This violates the law of conservation of energy. Lenz's law ensures that mechanical work must be done against the opposing magnetic force to sustain the induced current.

Lenz's Law Quick Reference

What Is Motional EMF and How Is It Calculated?

Motional EMF is the EMF induced when a conductor moves through a magnetic field. If a rod of length l moves with velocity v perpendicular to a uniform magnetic field B, the free electrons in the rod experience a Lorentz force, creating a potential difference.

Formula:

$$\varepsilon = Blv$$

For a rod making angle θ with the direction of motion:

$$\varepsilon = Blv\sin\theta$$

Motional EMF in a Rotating Rod

For a rod of length l rotating about one end with angular velocity ω in a field B perpendicular to the plane of rotation:

$$\varepsilon = \frac{1}{2}B\omega l^2$$

Power Dissipated in the Circuit

If the rod slides on rails with resistance R:

$$I = \frac{Blv}{R}, \quad P = \frac{B^2l^2v^2}{R}$$

This power equals the rate of work done by the external force maintaining the velocity — again confirming energy conservation.

Motional EMF problems in JEE Advanced frequently combine the sliding rod with a changing external circuit resistance. Always redraw the circuit at each stage and treat the rod as a battery of EMF = Blv with internal resistance = rod's resistance. This model solves 90% of such problems cleanly.

Self-Induction and Mutual Induction

What Is Self-Inductance?

When the current in a coil changes, it changes the magnetic flux through the coil itself, inducing a back-EMF. This property is called self-induction.

$$\varepsilon = -L\frac{dI}{dt}$$

L is the self-inductance (measured in Henry, H). For a solenoid of n turns per unit length, cross-sectional area A, and length l:

$$L = \mu_0 n^2 Al = \frac{\mu_0 N^2 A}{l}$$

Energy stored in an inductor:

$$U = \frac{1}{2}LI^2$$

What Is Mutual Inductance?

When two coils are placed near each other, a changing current in one (primary) coil induces an EMF in the other (secondary) coil. This is mutual induction.

$$\varepsilon_2 = -M\frac{dI_1}{dt}$$

M is the mutual inductance between the two coils. For two coaxial solenoids:

$$M = \mu_0 n_1 n_2 A l$$

Relationship Between M and L

For two coupled coils with self-inductances L₁ and L₂ and coupling coefficient k:

$$M = k\sqrt{L_1 L_2}, \quad 0 \leq k \leq 1$$

Eddy Currents — Causes and Applications

When a bulk conductor is placed in a changing magnetic field, currents are induced in the body of the conductor itself — not just along a defined path. These circulating currents are called eddy currents (or Foucault currents).

Causes of Eddy Currents

Eddy currents arise due to the same principle as Faraday's law — changing flux through the conductor body. Their magnitude depends on the rate of change of flux and the resistivity of the material.

Applications of Eddy Currents

• Magnetic braking in trains and roller coasters (contactless, smooth braking)
• Induction heating in industrial furnaces and induction cooktops
• Speedometers in vehicles (Weston eddy current type)
• Energy meters (electricity consumption measurement)

How Are Eddy Currents Minimised?

Eddy currents cause energy loss as heat. To minimise them, transformer cores and motor armatures are made of thin laminated sheets insulated from each other, reducing the effective area and hence the magnitude of eddy currents.

Topic-Wise Exam Weightage Table

Based on NTA JEE Main question paper analysis and NEET official question papers. Always verify the latest syllabus at ntaexam.com.

How to Revise Class 12 Physics Electromagnetic Induction Notes

• Write all the formulas of this chapter on one page and revise it before every test.
• Always find the sign and direction with Lenz’s law after finding the magnitude.
• Solve the exercises and compare your steps with the NCERT Solutions for Class 12 Physics, especially Chapter 6.
• For the Lorentz force and magnetic field concepts used here, revise with the NCERT Books Class 12 and the NCERT Solutions for Class 11 Physics.

Electromagnetic Induction Notes

The pages below are handwritten class 12 physics electromagnetic induction notes. Use them along with the formulas above for quick revision.

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

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